Vertical cavity surface emitting lasers capable of emitting long wavelengths are of interest in optical communication systems. In particular, emission of light having wavelengths near 1.3 .mu.m and 1.5 .mu.m has wide applications in fiber optic communications. Unfortunately, for a given wavelength, materials ideal for formation of the gain region of a vertical cavity surface emitting laser (VCSEL) are not always ideally suited for formation of the mirror regions of the VCSEL. For example, for light emission in the 1.2 .mu.m to 1.6 .mu.m wavelength range, the material which can be grown lattice matched to indium phosphide (InP) is ideal for gain region formation. However, material which is lattice matched to indium phosphide is undesirable for VCSEL mirror formation since it does not provide high reflectivity in the 1.2 .mu.m to 1.5 .mu.m wavelength range. Similarly, while the material lattice-matched to GaAs substrates makes highly reflective mirrors, it is not a good material choice for VCSEL gain region formation in the 1.3 .mu.m and 1.6 .mu.m wavelength range.
The reference "Continuous Wave GaInAsP/InP Surface Emitting Lasers with a Thermally Conductive MgO/Si Mirror", T. Baba, et al, Jpn. J, Appl. Phys., Vol. 33 (1994), pp. 1905-1909, describes a VCSEL which uses different materials for the gain region and mirror region formation. FIG. 1 shows an etched well VCSEL 100 such as is described in Baba, et al. The etched well VCSEL 100 shown in FIG. 1 is comprised of a gain region 102 formed on an indium phosphide substrate 104, an n-side mirror region 112 comprised of six pairs of SiO.sub.2 /Si layers, and a p-side mirror region 110 comprised of 8.5 pairs of (MgO/Si) layers. The mirror regions 110, 112 are formed by depositing dielectric films on the active region 102 and the indium phosphide substrate 104, respectively. Although, the dielectric mirror regions 110 and 112 provide high reflectivity which could not be accomplished by using semiconductor layers lattice-matched to an indium phosphide substrate, the dielectric mirrors 110, 112 provide poor thermal and no electrical conduction. Poor electrical and thermal conductivity of the mirror regions results in overheating of the VCSEL, negatively impacting device performance characteristics.
Alternatively, different materials for manufacture of the gain region and mirror regions of a VCSEL may be integrated by fusing a second mirror region material to a first material used for gain region formation. The gain region being previously deposited on the first mirror region using deposition techniques well known in the art. One example of such a structure is shown in the article by Babic', et al., "Optically Pumped all-epitaxial wafer-fused 1.52 .mu.m vertical cavity lasers," Electronic Letters, Apr. 28, 1994, Vol. 30, No. 9. Although the semiconductor mirror regions of the VCSEL structure described in Babic', et al. offer improved thermal and electrical conductivity compared to the insulating dielectric mirrors 110, 112 of Baba, et al., the Babic' laser is difficult to manufacture. Although the Babic' laser design is useable for operating at 1.5 .mu.m, it is probably not capable of CW high power operation at 1.3 .mu.m.
Another example of fusing a first mirror region comprised of a first material to a second material for forming the gain region comprised of a different material is described in the reference "Low Threshold Wafer Fused Long Wavelength Vertical Cavity Lasers," by Dudley, et al., Applied Physics Letters, Vol. 64, No. 12, 1463-5, Mar. 21, 1994. FIG. 2 shows a single fused VCSEL 200 as described by Dudley, et al. The VCSEL described in Dudley, et al. combines a semiconductor mirror region 212 with an alternating semiconductor/dielectric mirror region 210. Although the semiconductor mirror 212 offers improved thermal and electrical conductivity compared to the insulating dielectric mirror 112 shown in FIG. 1, the VCSEL 200 shown in FIG. 2 still has poor thermal and electrical conductivity through dielectric mirror 210. Further, the laser shown in FIG. 2 injects current at the edge of the device. Injecting current at the device edge instead of through the center of the device causes an increase the heat generated, decreasing laser performance. The laser performance is also decreased due to the poor overlap of the carrier profile and the optical mode profile. This could cause the laser to operate in multiple transverse modes which is a problem for communication systems and for stable fiber optic coupling.
An example of using wafer bonding techniques for LED formation is shown in U.S. Pat. No. 5,376,580. Referring to FIG. 8 of U.S. Pat. No. 5,376,580, for example, shows wafer bonding a first growth substrate 30 and a second substrate 48 to epitaxial layers 32-38. Wafer bonding for LED formation is typically used to bond a substrate material that is optically transparent to a LED active region formed of a different material.
A top or bottom emitting VCSEL in the 1.3 .mu.m and 1.5 .mu.m wavelength range which provides a high gain, high reflectivity, good thermal conductivity and good electrical conduction through both mirrors is needed.